I just ask them to exit AC.
Srini,
Bert gave a thorough description of several different processes. When you say "exit AC", are you sure they know the difference between "logging out" and "quitting" AC? This is relevant to your problem.
I have instructions to the staff to "QUIT" AC rather than "logoff" just incase I need to restart the server in the night for maint.
There is no harm in restarting the server even if they are just logged off and not quit.